The invention relates to the general field of magnetic read heads with particular reference to GMR structures for use in reading data recorded at densities in the 100Gb per sq. in. range.
Requirements on transducers for ultra-high recording densities (greater than 60 Gb/in2) place certain constraints on the properties of the read and write heads needed to achieve this. These fundamental constraints have a profound influence on the design and fabrication of the read/write transducers. To achieve extremely high recording densities, Giant Magnetoresistance (GMR) reader design has to be capable of very high linear bit density (BPI) and also very high track density (TPI). Consequently, GMR devices continue to be pushed to narrower track widths and to thinner free layers to maintain high signal output in spite of reductions in track width and reduced gap length.
Ultra-thin free layers as well as MR ratios are very effective to obtain high signal output. Ultra-thin free layers having moments equivalent to 37 xc3x85 NiFe (20 xc3x85 CoFe) made of a composite CoFexe2x80x94NiFexe2x80x94Cu layer, is capped with 10-20 xc3x85 Ta or TaO. The large GMR ratio obtained from such a very thin free layer is due to: (a) the Cu HCL (high conductivity layer) which improves the mean free path of a spin-up electron and maintains the mean free path difference between spin-up and spin-down electrons, and, (b) the bottom spin valve structure which provides very good specular reflection at the Ru/CoFe and CoFexe2x80x94NiFexe2x80x94Cu(free layer)/Ta or TaO interfaces.
There are other features that make the bottom spin valve most suitable for extremely high recording densities. It is shown that the thin free layer of the bottom spin valve is magnetically softer than that of the top spin valve. GMR magnetostriction in the top spin valve increases asymptotically with the reduced free layer thickness. For the bottom spin valve, the magnetostriction can be attenuated by increasing the CoFe thickness in a CoFexe2x80x94NiFe composite free layer. A thicker CoFe also improves the GMR ratio (Dr/r). One unique feature for the bottom spin valve is that the sensor longitudinal biasing can be made by a patterned exchange bias.
Spin valves with contiguous hard bias to achieve sensor stability are known to suffer amplitude loss due to the field originating from the hard bias structure. One approach that has been proposed to overcoming some of the amplitude loss and stability concerns has been to use a lead overlay design. In lead overlay design, MR sensor track width is defined by conductor lead edge while the contiguous hard bias junction is placed outside the conductor lead. The overlap length between the lead overlay and the hard bias junction should be less than 0.1 microns. This requirement imposes a great challenge to the photo-lithography.
An example of a bottom spin valve structure that is typical of the prior art is shown in cross-section in FIG. 1. Seen there is bottom magnetic shield 15 (commonly referred to as Si) which is coated with lower dielectric layer 17 (and commonly referred to as D1). Over this is seed layer 10 (typically nickel-chromium) which is, in turn, coated with pinning (antiferromagnetic) layer 14. Layer 13 is the pinned layer and layer 12 is the non-magnetic, electrically conductive layer. Layer 11a is the free layer, layer 11b being formed of the same material as layer 11a. Layer 19 is a capping layer while layer 114 is an anti-ferromagnetic layer that serves to provide longitudinal bias to the spin valve.
Continuing with FIG. 1, layer 110 is conductive material that serves for the formation of leads to the device. Layer 18 is the upper dielectric layer (commonly referred to as D3). Finally, layer 16 is the upper magnetic shield, which is commonly referred to as S2.
One key factor to improving BPI is to reduce the reader gap length (commonly called the shield-to-shield spacing). This is equal to the distance between S1 and S2 within the GMR sensor area in FIG. 1. For a 100 Gb/in2 recording density design, the sensor track width is 0.1 microns, and the gap length (S1/S2) is around 600 xc3x85. For a 300 xc3x85 thick GMR stack, even for a very thin D1 (about 140 xc3x85), D3 is less than 160 xc3x85 thick. The greatest concern with a very thin dielectric D1/D3 layer is (a) sensor to shield shorts and (b) dielectric breakdown.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,302,461, Anthony shows dielectric layers for MR heads including oxides of Ta, Hf, Zr, Y, Ti or Nb. The invention pertains to an MR (as opposed to a GMR) read head. The metal is deposited directly onto the MR plates and then allowed to oxidize.
Yamamoto et al. in U.S. Pat. No. 5,919,581 show a MR with a shield layer. Hsiao et al. in U.S. Pat. No. 5,999,379 and Kawano et al. in U.S. Pat. No. 5,432,734 disclose other MR structures with dielectric layers.
It has been an object of the present invention to provide a bottom spin valve magnetic read head suitable for use with ultrahigh recording densities, typically about 100 Gb/in2.
Another object of the invention has been that the separation between the magnetic shields S1 and S2 be less than 700 Angstroms while withstanding electrostatic breakdown between the GMR sensor and the shields.
These objects have been achieved in a Spin Valve structure that is a spin-filter, synthetic antiferromagnet bottom spin valve. A key novel feature is that the upper and lower dielectric layers D1, and D3, which are normally pure aluminum oxide, have each been replaced by a bilayer dielectric, each of which consists of aluminum oxide in contact with the shield layer and a layer of a high voltage breakdown material. For D1 this layer may be either tantalum oxide or tantalum nitride while for D3 our preferred material has been tantalum oxide. The addition of the two high breakdown layers allows the thickness of the upper and lower dielectric layers to be reduced without increasing the incidence of shorts associated with dielectric breakdown in D1 and/or D3.